1. Field of the Invention
The present invention relates to a turbofan engine of a type having a zero hub tip ratio fan structured such that a hub/tip ratio in the first-stage moving blade of a fan can be set to approximately 0.
2. Description of Related Art
As outlined in FIG. 1, a high-bypass turbofan engine that is one of jet engines is structured such that a fan 1, a compressor 2, a combustor 3 and a turbine 4 are arranged in an axial direction (shown by an axis O) of the turbine in such order from a front side. Air is taken from an inlet (an air intake) 5 in a front surface by rotation of the fan 1 driven by the turbine 4. Most of the air is injected from a bypass duct 6 so as to generate a driving force, and a part of the rest of the air is compressed by the compressor 2 driven by the turbine 4. Fuel is burnt in the combustor 3 by the compression air, and the turbine 4 is driven by the high-speed combustion air flow generated at the time of the combustion.
In this kind of high-bypass turbofan engine, there has been known that an average speed of an exhaust gas can be decreased by making the bypass ratio large, thereby contributing to reduction of specific fuel consumption and reduction of an exhaust gas noise.
In this case, the fan 1 in the prior-art high-bypass turbofan engine is structured such that a base portion (end portion at rotational center side) of the first-stage moving blade (the up-front fan moving blade) 7 is embedded in an outer circumferential surface of a spinner 8 rotationally driven by the turbine 4. For this reason, certain degree of hub/tip ratio (a ratio of a hub diameter to a tip diameter facing to the inlet 5) is necessary. The hub/tip ratio is generally set to about 0.3.
Accordingly, when the bypass ratio in the high-bypass turbofan engine is enlarged to achieve a lower specific fuel consumption and a lower noise, there is a problem that not only a diameter of the first-stage moving blade 7 of the fan but also an inner diameter of a casing 9 surrounding the first-stage moving blade of the fan becomes large, causing increase of an engine weight.
In view of the above problem, the applicant of the present invention has proposed a type of a turbofan engine equipped with a fan (hereinafter, refer to as a zero hub tip ratio fan) 10 structured such that a hub/tip ratio can be set to 0, as shown in FIG. 2 (for example, refer to Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-27854).
As for this zero hub tip ratio fan 10, a front side of a base portion of a rear moving blade portion 11a arranged in an outer periphery of the spinner 8 (rotationally driven by the turbine 4 in the same manner as the first-stage moving blade 7 of the fan shown in FIG. 1) is spirally extended toward an axial position of the front end portion so as to form a spiral blade portion 11b. Thereby, the first-stage moving blade 11 of the fan is formed at a front portion of the spinner 8 with the hub diameter set to 0.
Accordingly, in the turbofan engine of the zero hub tip ratio fan type, since the air can be sucked from the front surface of the spinner 8 by the spiral blade portion 11b, and is compressed so as to be supplied to the rear moving blade portion 11a, it is possible to set an entire area in front of the engine as an air inflow area of the first-stage moving blade 11 of the fan. Therefore, since it is possible to increase a mass flow rate of the first-stage moving blade 11 of the fan without enlarging the diameter of the fan and the inner diameter of the casing, it is possible to keep the diameter of the fan small even in the case where the bypass ratio is enlarged, and it is possible to reduce weight of the engine.
However, in the prior-art zero hub tip ratio fan type turbofan engine as shown in FIG. 2, the first-stage moving blade 11 of the fan in the zero hub tip ratio fan 10 is structured such that the spiral blade portion 11b and the rear moving blade portion 11a are integrated so as to be smoothly connected. For this reason, in order to arrange the first-stage moving blade of the fan having such a blade shape in the front portion of the spinner 8, it is necessary to employ a so-called brisk structure in which the front portion of the spinner 8 and the first-stage moving blade 11 of the fan are integrated. In this case, for example, when a part of the first-stage moving blade 11 of the fan is broken due to a bird strike or the like, it is necessary to replace an entire of the brisk structure. Accordingly, there is a disadvantage from the standpoint of maintainability.
Further, in the case of manufacturing the front portion of the spinner 8 and the first-stage moving blade 11 of the fan as the brisk structure, it is necessary to perform an integral molding by a cutting or the like. However, since the brisk structure is large scale structure including an entire of the first-stage moving blade 11 of the fan and the front portion of the spinner 8, manufacturing costs become high.